The invention relates to an electromagnetic actuator for moving a contact into a switched-on or switched-off state, comprising a contact-actuating rod, which is displaceable in the longitudinal direction between a first position, corresponding to the switched-off state, and a second position, corresponding to the switched-on state, a core, which is made of magnetizable material and is attached to the contact-actuating rod, a switch-on coil, which interacts with the core, a pole piece, which is made of magnetizable material and of which that face which is directed towards the core, in the first position of the contact-actuating rod, is arranged at an air-gap distance from that surface of the core which runs perpendicular to the direction of displacement and, in the second position, bears as closely as possible against the said core surface, a yoke, which is made of magnetizable material, for closing the magnetic flux circuit of the switch-on coil through the pole piece and the core, a permanent magnet device for maintaining the contact-actuating rod in the first position and a spring which preloads the contact-actuating rod, in its second position, towards the first position. An actuator of this kind is known from British patent application GB-A-2,289,374.
There are a number of initial considerations which are important for electromagnetic actuators and deal with the switching safety and the service life of a vacuum switch employed in medium-voltage distribution networks:
1. Switching on is to take place quickly, so that damage caused by the contact surfaces burning in as a consequence of flash-over is limited. PA0 2. Maintaining the switched-on state is to be achieved with a sufficiently high contact pressure, because otherwise excessive contact resistance will lead to dissipation between the contacts, which may cause them to become welded together. This occurs primarily under high short-circuit currents. PA0 3. Opening of the contacts is to take place with a high impulse level, in order to break open any contacts which may have welded together. PA0 4. Opening of the contacts is also to take place at high speed, in order to limit the extent to which the contact surfaces burn in as a result of the arc produced. PA0 5. For the sake of operational reliability of the drive mechanism, it should be sought to keep the number of components as low as possible. The failure of a switch can generally be attributed to a failed drive mechanism. PA0 6. In order to be able to make maximum use of the switching capacity available, it is sometimes desirable to switch off at a specific moment in the current or voltage curve. In a three-phase system, this switching moment may differ for each phase, and the switching pattern may also vary each time depending on the conditions.
In the past, the first five points for consideration have been met by mechanical systems which acted on the basis of energy stored in springs. These systems also allow constant delay times to be achieved. Nevertheless, these drives still fail on occasions.
The abovementioned British patent application relates to a bistable actuator which operates with a set of permanent magnets, a coil and a spring. As soon as a current is fed to the coil, the contact moves into the closed or switched-on state. The field of the coil generated by the current is oriented in the same direction as the magnetic field of the permanent magnet. The total magnetic force brings about easy excitation, only a little current being required in order to move the contacts into the switched-on state. In the switched-on state, the spring is compressed and the actuating rod is held in place by the permanent magnets. The field of the permanent magnets exerts a force on the actuating rod which is greater than the force of the spring and is oppositely directed to the spring force. As soon as the switched-on state of the contacts is reached, the electric current through the coil can be interrupted.
In order to move the contacts into the open or switched-off state, a pulse of electric current is fed to the coil, generating a field which is oppositely directed to that of the permanent magnets. The force on the actuating rod generated by the field of the permanent magnets is thus partially eliminated, so that the actuating rod, on the one hand, is pressed by the energy stored in the spring into the position corresponding to the switched-off state and, on the other hand, is still slowed down to some extent by the residual force generated by the permanent magnets.
Therefore, this known actuator does not fulfil the demands imposed by the inventor that switching off should be quick. This can be attributed to the fact that the magnetic flux, when moving these contacts into the switched-off state, is reduced too slowly in the switched-on state of the contacts.
The switch-on time for an actuator is defined as the time from the start of excitation of the switch-on coil until the point at which the contacts actuated by the actuator come into contact with one another. In the case of actuators for actuating contacts which are suitable for switching high powers, the switch-on time is very great and is not reproducible. Owing to the high self-induction of the switch-on coil of the actuator, the current rises slowly to the maximum achievable level. If, during this build-up of the current, the tensile force of the actuator is sufficiently great to overcome the opposing force occurring in the switched-off state (as a result of, inter alia, friction, switch-off spring, temperature, etc.), the mobile part of the actuator, i.e. the contact-actuating rod, begins to move. The moment at which this happens depends, inter alia, on tolerances in current intensity and friction. The switch-on time, i.e. the time from which the current is switched on until the contacts actually close, is difficult to predict and the switch-on time is therefore variable and not reproducible.